coverage: Don't rely on the custom traversal to find enclosing loops

[Zalathar]

This opens up the possibility of modifying or removing the custom graph traversal used in coverage counter creation, without losing access to the heuristics that care about a node's enclosing loops.

Actually changing the traversal is left for future work, because this PR on its own doesn't change the emitted coverage mappings at all.

[rustbot]

r? @saethlin

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[Zalathar]

Clarified that dominator order doesn't have to be reverse postorder; that's just what it currently is (diff).

[Swatinem]

lgtm

[Swatinem]

just a side note here, the Rust 1.82 release notes mention that we now have dedicated use<> syntax to replace Captures, and this eventually becoming the default in Rust 2024.
Though I believe + '_ would also work here, as the &self lifelime is the only one captured here?

I also don’t really know what the current conventions for compiler code are related to all this.

[Zalathar]

Oh, is that stable now? Nice.

I think I'm going to stick with Captures for now. After the compiler switches over to Rust 2024, someone will probably go through and try to rip out unnecessary uses of Captures, and it will be easier to find and migrate this code if it matches the prevailing style in the rest of the compiler.

[saethlin]

@bors r+

[bors]

📌 Commit 19b2142 has been approved by saethlin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 19b2142 with merge e454c45...

[bors]

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing e454c45 to master...

[rust-timer]

Finished benchmarking commit (e454c45): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 1.4%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.3%, 1.5%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[1.3%, 1.5%]	3

Cycles

Results (primary 1.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.2%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.2%]	3

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 783.844s -> 783.471s (-0.05%)
Artifact size: 333.65 MiB -> 333.71 MiB (0.02%)